This chapter will be a veritable "parade of taxa", as we start tracing
the evolution of vertebrates and the derived characters that distinguish
them from the chordates that we discussed in the last lecture.

General characteristics of vertebrates

Vertebrates may be characterized by 12 general derived characteristics.
You should become very familiar with these traits, and identify how they
are expressed in the vertebrates you will see in lab.

1. Bilateral symmetry

2. Two pairs of jointed locomotor appendages, which can include fins
(pectoral and anal/dorsal fins, as well as the forelimbs and hindlimbs).

3. Outer covering of protective cellular skin, which can be modified
into special structures such as scales, hair and feathers

4. Metamerism found in skeletal, muscular and nervous system. This was
described in a previous lecture - structures can include ribs, vertebrae,
muscles and ganglia/peripheral nerves.

5. Well-developed coelom, or body cavity completely lined with epithelium
(cellular tissue), that may be divided into 2 to 4 compartments.

Or based on their embryonic characteristics
Anamniotes - vertebrates that lack an amnion, or extraembryonic
membrane that surrounds the embryo and encases it in amniotic fluid. Includes
Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia.

Don't let these different terms confuse you! They are all ways of distinguishing
taxa based on primitive versus derived traits. Use them to help you memorize
and classify the taxa we will be talking about.

Tracing vertebrate evolution through the fossil record

Keep in mind that the evolutionary relationships among the different
taxa that we are discussing have been determined from the fossil record.
Vertebrates, among all species of animals in the world, have the best fossil
record.

Reasons include the presence of hard parts, such as bones and scales.
The exception to this are the cartilaginous fish, because cartilage does
not fossilize well, and the birds, due to the fact that they have hollow
bones that can be crushed and lost. In contrast, the best fossil record
among the vertebrates exists among the large mammals, whose bones are preserved
well as fossils.

Superclass Agnatha

The Agnatha are in some texts referred to as a class, and in others
as a superclass. In general this group shares the common characteristics
of:

• no jaws

• no paired appendages

• a completely cartilagenous skeleton

• a single nostril

• 6 - 14 external or concealed gill slits

• a persistent notochord

• a two-chambered heart

Because the fossil record is very poor for these species, it is unclear
whether the two Agnathan groups should be described as classes or orders.

Ancestral forms of this class were the Ostracoderms, which are extinct,
but were heavily-armored on their heads and trunk. The ostracoderms were
believed to be detritus feeders, because of their jawless mouths.

Extant Agnathans include two groups, called cyclostomes, because
of their circular mouths:

• larvae are called ammocoetes, resembling the amphioxus - primarily
detritus feeders until they metamorphose into adults, sometimes after 6
or 7 years as a larva.

The remaining vertebrate orders are Gnathostomes (possess true jaws).
Evolutionary studies have shown that in most cases the jaw is modified
from one of the gill arches that were used to support gills in more primitive
species.

Evolution of jaws represents an advancement in morphology, expanding
the function of the mouth to a wider range of potential prey types. Thus,
the jaws are an example of a derived structure that is more generalized
than its ancestral form.

• claspers present in males, internal fertilization, ovoviviparous
(egg contained within the uterus, where the young develop and then hatch
as miniature adults) or viviparous (embryos develop internally and
then emerge as a miniature adult)

• modern species present by end of Mesozoic

Order Squaliformes - true sharks

• almost purely predaceous/marine

• heterocercal tailfin - caudal fin is longer on the dorsal side than
on the ventral side

Subclass Sarcopterygii: species previously believed to be extinct,
such as the coelacanths and lungfish.
• fleshy lobed fins so that fin rays do not articulate directly to
girdles

• internal and external nares

• many retain the heterocercal tail

• the coelacanth is represented by a single species that lives off the
Comoro Islands near Madagascar

The rise of the Tetrapoda classes and the movement from water to land represents
one of the major evolutionary events in the history of vertebrates. New
structural designs were required to make the transition to land in order
to cope with increased oxygen levels, decreased water supply, more fluctuating
ambient temperature, and slight changes in the way sensory information
is obtained.

Class Amphibia - Amphibians

• arose from Crossopterygian, Rhipidistian ancestors

• three extant orders, two extinct subclasses

• lungs and skin used as adult respiratory organs

• gills present in larvae, retained into adulthood in some neotinic
forms (salamanders)

• heart with two atria and one ventricle - "three chambered"

• skin is naked or with bony dermal elements

• ectothermic - must regulate body temperature by moving to different
microclimates within its environment

• group includes smallest terrestrial vertebrates up to some 5’ in length

• name implies continued tie to water - eggs must be laid in water or
at least in very moist environment; young develop as gill breathing, water-dwelling
tadpoles

• embryos lack an amnion, but eggs are laid in a jelly-like protective
coating

Order Urodela (Caudata) - salamanders• tail maintained throughout life

• limbs 1 -2 "normal" pairs

• elongated trunk and long tail

• can retain larval characteristics (flattened, shovel-shaped head,
fleshy tail, external gills) in adult forms (paedomorphic) - the
result is a sexually mature individual with many other body parts in the
larval or juvenile condition (neoteny)

• arose in Cretaceous, great radiation of insectivore-like ancestors
during Cenozoic

Final note: We will be using the terminology and phylogenies developed
in this lecture as we go through the different systems. Try to become comfortably
familiar with this phylogeny, as I will be referring to it later. Because
this is comparative anatomy, the focus of this course will be on how different
systems developed in different taxa, without emphasis on any single taxonomic
group (such as humans).

General Definitions:

Adaptive radiation - evolutionary process in which descendants from
an ancestral species multiply and diverge to occupy many different habitats
and modes of life

Agnatha - jawless vertebrates, including Myxini, Cephalaspidomorpha

Amniotes - vertebrates that possess an amnion or extraembryonic membrane
that surrounds the embryo and encases it in amniotic fluid, including Reptilia,
Aves, and Mammalia